Magnetic induction has been a time-honored and reliable method of measuring the speed of a rotating gear. Common applications include timing sensors for ignition control of internal combustion engines in automobiles, motorcycles, etc., as well as for monitoring engine RPM and vehicle speed. These sensors are limited by the fact that they can only function well over very small airgaps, typically 1-2 mm at best, requiring very tight tolerances between the sensor and the target gear, increasing the chances of sensor failure. Increasing the airgap would reduce tolerances required between the sensor and the target gear, resulting in significant cost savings for the manufacturers of such systems as ABS braking systems for automobiles.
Magnetic induction works on the principle that a magnetic field relative to a coil will induce a voltage into a coil which is proportional to the time rate of the relative motion. The target gear, which acts as a rotor, is generally a toothed gear made of ferromagnetic steel or it can be made of non-ferrous material with magnets embedded in it. Generally, existing sensors comprise a coil of a certain number of turns wound around a ferrous steel pole piece or ferrite bead. A magnet is placed in near proximity to the target end of the sensor. The magnet at the end of the wire ferrite bead or pole piece, the bead acting as a flux concentrator, conducts a magnetic flux path from the end of the sensor, creating a magnetic field in front of the sensor. When a ferromagnetic tooth from a rotating gear passes through this magnetic field, the resulting changes creates a cmf in the coil. With increasing speed the number of pulses per unit time increases proportionally. The increasing rate of change of magnetic flux also produces increasing pulse amplitude.
In practice, with enough windings on the coil for maximal sensitivity, an inductive sensor can sense the first tooth of a target gear as it begins to move at very slow speeds, but again, only over a very small airgap. More sensitive inductive coil sensors can be produced by increasing the number of windings, but this rapidly renders the sensor too unwieldly in size for many applications, particularly in vehicles. The present invention provides for small sensor size coupled with increased sensitivity for larger airgap use.
Based on the foregoing it is the general object of the present invention to provide an inductive sensor that improves upon or overcomes the problems and drawbacks associated with prior art sensors.